The Role of NKG2D Signaling in Inhibition of Cytotoxic T-Lymphocyte Lysis by the Murine Cytomegalovirus Immunoevasinm152/gp40

Abstract

Three proteins encoded by murine cytomegalovirus (MCMV)-gp34, encoded by m04 (m04/gp34), gp48, encoded by m06 (m06/gp48), and gp40, encoded by m152 (m152/gp40)-act together to powerfully impact the ability of primed cytotoxic CD8 T lymphocytes (CTL) to kill virus-infected cells. Of these three, the impact of m152/gp40 on CTL lysis appears greater than would be expected based on its impact on cell surface major histocompatibility complex (MHC) class I. In addition to MHC class I, m152/gp40 also downregulates the RAE-1 family of NKG2D ligands, which can provide costimulation for CD8 T cells. We hypothesized that m152/gp40 may impact CTL lysis so profoundly because it inhibits both antigen presentation and NKG2D-mediated costimulation. We therefore tested the extent to which m152/gp40's ability to inhibit CTL lysis of MCMV-infected cells could be accounted for by its inhibition of NKG2D signaling. As was predictable from the results reported in the literature, NKG2D ligands were not detected by NKG2D tetramer staining of cells infected with wild-type MCMV, whereas those infected with MCMV lacking m152/gp40 displayed measurable levels of the NKG2D ligand. To determine whether NKG2D signaling contributed to the ability of CTL to lyse these cells, we used a blocking anti-NKG2D antibody. Blocking NKG2D signaling did affect the killing of MCMV-infected cells for some epitopes. However, for all epitopes, the impact of m152/gp40 on CTL lysis was much greater than the impact of inhibition of NKG2D signaling. We conclude that the downregulation of NKG2D ligands by MCMV makes only a small contribution to the impact of m152/gp40 on CTL lysis and only for a small subset of CTL.All cytomegaloviruses (CMVs) interfere with the major histocompatibility complex (MHC) class I pathway of antigen presentation. Although the biological ramifications of this interference remain unclear, the variety of molecular mechanisms employed by different CMVs to attack MHC class I is impressive. Murine CMV (MCMV) contains three viral genes whose products interfere with MHC class I: m04/gp34, m06/ gp48, and m152/gp40 (37). This interference has been demonstrated by their ability to impair the lysis of infected targets by cytotoxic T-lymphocytes (CTL) (17,31). m06/gp48 binds MHC class I and diverts it to lysosomes for destruction (33). m152/gp40 causes nascent MHC class I to be retained in the endoplasmic reticulum cis-Golgi intermediate compartment (6,34,38). m04/gp34 binds some MHC class I and escorts it to the cell surface; in different circumstances, m04/gp34 can either increase or decrease susceptibility to CTL lysis (17,21,30,31). Both m06/gp48 and m152/gp40 reduce the cell surface expression of MHC class I, whereas m04/gp34 either has no impact on cell surface MHC class I or slightly elevates the levels (22, 31, 35). We recently assessed the relative impacts of the three MHC class I immune evasion genes on the ability of CTL specific for 15 H-2 b -restricted epitopes to lyse infected cells (31). This study revealed that the three genes act pow...

Conserved Amino Acids within the Adenovirus 2 E3/19K Protein Differentially Affect Downregulation of MHC Class I and MICA/B Proteins

“…By contrast, Ad appears to use solely E3/19K to avoid recognition by both CTL and NK cells (15). In this respect, E3/19K is most similar to m152/gp40 of mouse cytomegalovirus that also targets both MHC-I and NKG2D ligands (27,73). The involvement of multiple target molecules that interact with receptors on different cell types (NK and CTL) makes it difficult to predict the effect of viral subversion of innate and adaptive cytotoxic lymphocytes.…”

“…The involvement of multiple target molecules that interact with receptors on different cell types (NK and CTL) makes it difficult to predict the effect of viral subversion of innate and adaptive cytotoxic lymphocytes. For example, NKG2D may or may not contribute to activation of CD8 T cells (73,74). Infection with Ads and Ad vectors that lack E3/ 19K has been shown to provoke NK cell lysis (15,28,75).…”

Epitope-specific in vivo protection against cytomegalovirus disease by CD8 T cells in the murine model of preemptive immunotherapy

“…In addition, as we have shown previously, IE1-CTLL completely failed in being sensitized by infected MEF derived from BALB/c-H-2 dm2 mice in which the IE1-epitope presenting MHC class-I molecule L d is not expressed due to a deletion in the MHC locus [7]. Another argument against a critical role for NKG2D-RAE-1-mediated sensitization of CD8 T cells is provided by recent data from Ann Hill's group demonstrating that blockade of this interaction with anti-NKG2D antibody had for some epitopes a detectable but altogether little impact on CTL activity [19]. This is in accordance with the current view that NKG2D-RAE-1 interaction does not independently lead to signaling in NKG2D-expressing T cells, but that NKG2D may act as a costimulatory molecule enhancing TCR-mediated, epitope-specific sensitization [20].…”

MHC class I immune evasion in MCMV infection

“…A number of studies have shown that MCMV-speciWc CD8 T cells can kill cells infected with a mutant virus lacking one or more of the MHC class I immune evasion genes, but not cells infected with WT-MCMV [18,53,55,57,59,64,65]. It should be noted that most comparisons between mutant and WT-MCMV infection utilize a WT-BAC MCMV, which was generated using the Smith strain and bacterial artiWcial chromosome (BAC) technology [56].…”